A broad range of industrial processes require the separation and recovery of metal from metal-containing material. Of particular importance is the separation and recovery of uranium from uranium-containing material. Uranium-containing material is generated as a byproduct of numerous processes, mostly associated with the nuclear power industry. Two examples of waste materials that contain significant quantities of uranium are spent nuclear fuel and incinerator ash from facilities that make nuclear fuel. Due to its toxicity and potential value, recovery of uranium from these and other waste materials is desirable.
The PUREX (Plutonium and Uranium Recovery by Extraction) process currently is the most commonly used process for separating uranium from uranium-containing material. By this process, the uranium-containing material first is dissolved in nitric acid to form a uranyl nitrate solution. The uranium in this solution then is separated by an organic solvent, such as tributylphosphate (TBP) mixed with a diluent, such as dodecane. Subsequent liquid-liquid extractions further purify the uranium.
The primary drawbacks of the PUREX process are cost and waste generation. The PUREX process, for example, involves numerous liquid-liquid extractions, which increase the cost of the process and increase the amount of liquid waste. The nitric acid dissolution step generates gaseous oxides of nitrogen that must be scrubbed from the off gas. This scrubbing step generates additional dilute nitric acid liquid waste. In addition, residue left over after the nitric acid dissolution step often contains residual nitric acid and requires treatment before disposal.
The environmental and economic costs of the PUREX process vary depending on the concentration of uranium in the starting material. When nitric acid is used to dissolve materials with high concentrations of uranium, such as spent nuclear fuel rods, the resulting uranyl nitrate solution is relatively concentrated. In contrast, when nitric acid is used to dissolve materials with lower concentrations of uranium, such as incinerator ash, the resulting uranyl nitrate solution is less concentrated. More extensive liquid-liquid extraction is required to separate uranium from low-concentration uranyl nitrate solutions than is required to separate uranium from high-concentration uranyl nitrate solutions. Unfortunately, known processes to concentrate the uranyl nitrate solution before solvent extraction are not practical.
There is a need to recover uranium and other metals from metal-containing materials at a lower cost and with less waste generation. This need is especially strong for the recovery of uranium from starting materials with low-to-moderate concentrations of uranium. Incinerator ash is one example of such a material. Factories that use uranium typically incinerate all of their combustible waste after it has been contaminated by uranium. This combustible waste can include, for example, packaging, protective suits and filters. The ash left over after burning this waste can contain various concentrations of uranium depending on factors such as the level of contamination and the presence of non-combustible contaminants other than uranium. Incinerator ash from facilities that manufacture nuclear fuel typically contains from about 5% to about 30% uranium. Currently, there are vast stockpiles of uranium-containing incinerator ash waiting for treatment or disposal and more is produced every day. Alternatives to the PUREX process are desperately needed.
Extraction with carbon dioxide maintained in liquid or supercritical form by the application of high pressure has been suggested as a more environmentally benign and potentially less expensive approach to metal recovery. Relevant references on this type of extraction include Samsonov, M. D.; Wai, C. M.; Lee, S. C.; Kulyako, Y.; Smart, N. G. Dissolution of Uranium Dioxide in Supercritical Fluid Carbon Dioxide. Chem. Commun. 2001, 1868-69 (“Samsonov”) as well as U.S. Pat. Nos. 5,356,538, 5,606,724, 5,730,874, 5,770,085, 5,792,357, 5,840,193, 5,965,025, 6,132,491, 6,187,911, and U.S. Published Patent App. No. 2003/0183043 (“the Wai patent documents”), which are incorporated herein by reference. Collectively, Samsonov and the Wai patent documents disclose several variations of extraction with a liquid or supercritical fluid solvent, including the dissolution of tetravalent uranium dioxide with an acid-base complex including tributylphosphate and nitric acid.
The inventors of the present disclosure recognized a need for methods and systems specially designed for the practical application of cleaner and more efficient extraction technology to the recovery of metals, such as uranium, from metal-containing materials.